1. Field of the Invention
The present invention generally relates to integrated circuit testing and, more particularly, to testing phase rotators to ensure the integrity of phase rotator tap weights used to produce an output signal with a desired phase.
2. Description of the Related Art
Phase rotators, also known as phase interpolators, are used to construct an output signal having a phase that is related to the phase of a second signal in some desirable way. Phase rotators are often used in serial data transmission and receiving circuitry as a component for aligning a sampling clock to recover serial data. Phase rotators typically generate an output signal having a phase with a known relationship to the serial data. The output signal is typically generated from a mix of incoming signals having defined offset phase relationships (commonly referred to as phasors).
For example, as illustrated in FIG. 1, a phase rotator 100 may mix four phasors 102 with relative phase offsets of 0 degrees, 90 degrees, 180 degrees, and 270 degrees, in N discrete combinations (by applying different weights to each) to create an output signal with N discrete phases between 0 and 360 degrees. Weights to be applied to each phasor 102 may be specified by a weight code 104 (illustratively, a string of 32 bits).
As illustrated in FIG. 2, which lists exemplary phasor weights and resultant output phases, the entire unit circle (0-360 degrees) can be achieved by mixing various weights of either 2 or 3 of the input phasors 102. The table shows output phases at 45 degree increments. At these points, the phasor weights are always either off or on fully to create the phases as shown. In between these points there is an interpolation applied to create the intermediate phases.
The weighting function may be fairly close to linear. For example, at 45 degrees the 0 and 90 degree phasor weights are on full and the 180 and 270 weights are off. For the next phase step (e.g., about 51 degrees, assuming 64 phase steps of approx. 6 degrees each) the 0 weight would be decreased slightly from its max value, pulling the output phase toward 90. However, during manufacturing of integrated circuits, there can be defects that could produce opens or shorts causing one or more of the tap weights to be ineffective. If a defective tap weight exists, the mixing operation will produce output phases different from those desired.
Unfortunately, in most test systems, and in most on chip built in self test (BIST) algorithms, clock and data signals are all produced from the same reference clock signal. This results in a relatively high probability with normal manufacturing process variations that delays in the clock and data signal paths may be such that the defective tap weight in the phase rotator might be in a phase quadrant (e.g., 0, 90, 180, or 270) that is not required to produce the desired phase.
Consequently, the chip may function correctly through BIST or manufacturing test operations, but fail when put into the field where it is exposed to different data paths and different operating environments bringing the defective tap into play. In other words, the phase rotator might have to adjust its resultant phase to include the defective tap weight and the incorrect rotator output phase could produce errors in serial data reception.
Accordingly, what is needed is a technique for testing phase rotators to ensure defective tap weights are detected.